The WDM technology is still developing very fast. The use of optical amplifier in WDM network is common, but it makes the gain distribution of spectrum unequal. To make each channel of WDM having optical power control function can release the problem of optical signal noise ratio (OSNR) decreasing caused by disequilibrium of the optical power, which is brought about by optical signal transmission, multiplexing, and demultiplexing. The combination of separated WDM and optical power control device for each channel form VMUX (VOA(variable optical attenuator)+MUX) or VDMUX (VOA+DEMUX), and VMUX can dynamically equalize the optical power in terminal MUX or OADM devices. With regard to VDMUX, the optical power automatic gain control prior the terminal receiver can be achieved. At present optical power control technology related to discrete component mainly includes mechanical type, Planar Lightwave Circuit (PLC) technology, microelectronic mechanical system, and liquid crystal technology etc. The common feature of these technologies is that the attenuation of the optical power can be controlled, but in terms of the application of the multichannel WDM system (for example, 40 channels commonly used), the disadvantage of high cost and large size exists. On one hand, the input and output for each channel requires to be coupled by optical fibers, which increases cost in two aspects. The first is about material cost, because a pair of V-groove pigtails is needed for the Planar Light wave Circuit technology, and a pair of collimators is needed for the micro-optics method. The second is about coupling cost, since the optical path of the optical devices used in optical communication is extremely precise and sensitive, the coupling cost accounts for more than half of the cost of general optical devices. On the other hand, the additional use of coupling pigtails or collimators increases the size and insertion loss of the devices.
As above mentioned, conventionally, VMUX is formed by combining VOAs for each discrete single channel or a VOA array and a MUX/DMUX, but it has the disadvantage of large size, high cost, and low integration level. FIG. 1 shows an overall schematic diagram of a conventional optical-power adjustable WDM. The optical-power adjustable WDM includes an input fiber waveguide 1, a variable optical attenuator 2, an optical power detecting PIN 3, a WDM wavelength division multiplexer 4, an output waveguide 5, and a control panel 6, and the optical power detecting PIN 3 is optional. This solution combines discrete VOAs and a MUX/DMUX, but has the defects that the formed VMUX/VDMUX is bulky; the variable optical attenuator for each channel is coupled to corresponding channel of the WDM by soldering, which will incur additional loss and the soldering joint need to be protected.
In order to reduce the cost and the size, WDM has to be integrated with optical power control device as one component. There are two kinds of mainstream WDM technologies relating to it, that is, Planar Lightwave Circuit (PLC) technology and micro-optics technology, the integration of optical power control and WDM requires putting the optical power control device into the WDM optical path directly. In terms of the PLC technology, such product has existed, for example, those provided by Bookham and Hitachi, but the relevant polarization dependent loss is large because of the limitation of the PLC Technology principle, while for the micro-optics technology, the integration encounters great problems, because the conventional VOA is large in size compared with WDM, as a result, there is no scheme proposed to achieve the optical power control of WDM by integrating an optical attenuator chip into the optical path of the WDM.